The inventors had previously disclosed the first known case where a patient who had been HIV positive stopped treatment without immediate rebound of the viral population in the blood. In U.S. Ser. No. 08/812,515, filed Mar. 7, 1997, Method of Inhibiting Human Immunodeficiency Virus by combined use of Hydroxyurea, a nucleoside analog, and a protease inhibitor, which is incorporated by reference as if set forth in full, it was disclosed that a triple drug combination including hydroxyurea, a reverse transcriptase inhibitor, and a protease inhibitor, can drive the virus to undetectable levels in both the blood, and more significantly, the lymph nodes, according to the most sensitive tests then available. At the time, it was supposed that the triple combination derived much of its potency from the protease inhibitor.
As recently as a year ago, drug cocktails which did not use hydroxyurea such as a triple drug combination involving the use of AZT, 3TC and protease inhibitors had been suggested for the treatment of HIV-1 infection and eradication of the virus. The efficacy of this combination was thought to originate from the potency of the protease inhibitors and the mechanism of action of the AZT/3TC combination in inhibiting the rebound of resistant mutants. And indeed, many patients experienced impressive drops in viral load in the bloodstream, so that virus was undetectable, with virus becoming undetectable for some patients in as little as eight to sixteen weeks. Viral load, measured as HIV-1 RNA is the best available indicator of disease progression and reduced concentration of HIV-1 in various tissues and fluids in response to antiretroviral therapy, and is predictive of improved prognosis (Mellors, J. W. et al. Science 272(5265) 1167-1170, 1996). Viral load in the blood is more conveniently determined than viral load in other tissues.
Despite their promise, protease inhibitors are new drugs which must be explored in detail before they are marketed. As a result, they are expensive enough to be impractical for many patients. Most protease inhibitors must be taken on a very exacting schedule, or they will lose their effect. The drugs do not always elicit a patient response (defined as a significant drop in plasma viral load). Neither the protease inhibitors nor 3TC easily penetrate to certain organs such as lymph nodes and the brain, and the combination of protease inhibitor, AZT and 3TC apparently does not completely eradicate HIV-1 in macrophages or in quiescent cells, which are major reservoirs of HIV-1. Further, patients who have interrupted therapy using AZT, 3TC and protease inhibitors and then rebounded cannot be as effectively treated with either the same combination or the same combination with another protease inhibitor because they develop resistant mutants. Many times the resistance to one protease inhibitor extends to others. There currently exists a pool of patients who, having used a protease inhibitor in the past, can no longer benefit from the newer protease inhibitors. Finally, the protease inhibitor-containing combinations without hydroxyurea have shown at best, response rates of 80-90% and 53% "failure"--a combined figure including people who never responded to therapy, those who could not tolerate side effects, those who responded initially but later saw a return of detectable virus, and those who had difficulty adhering to the strict dosing regimens required by the drugs. See Project Inform Perspective 23:1-3, November 1997.
One explanation for the eventual rebound of viral load after apparently successful treatment may be found in Identification of a Reservoir for HIV-1 in Patients on Highly Active Retroviral Therapy, Finzi, et al., Science Magazine 278 (5341):1295. In a study of 22 patients successfully treated with highly active anti-retroviral therapy for up to 30 months, a highly aggressive new test method found replication-competent virus was routinely recovered from resting CD4+ T lymphocytes. The frequency of resting CD4+T cells harboring latent HIV-1 was low, 0.2 to 16.4 per million cells, and in cross-sectional analysis, did not decrease with increasing time in therapy.
Hyroxyurea has been widely used over the last three decades for the treatment of leukemia, sickle cell anemia, and has more recently been suggested for use in the treatment of HIV infections. EPO patent publication 94918016.0 filed May 17, 1994 and corresponding to U.S. Ser. No. 08/065,814, filed May 21, 1993, which is incorporated herein as if set forth in full, describes the in vitro data suggesting that it would be worthwhile to try administration of hydroxyurea in combination with ddl in human volunteers. The data shows that hydroxyurea used alone had an antiviral effect, and that ddl was the more potent of the two when each were used alone in vitro. When the combination was tried in human beings, a therapeutic effect was observed in that the viral load in the blood, or plasma viremia, was reduced, although the amount of virus in the blood never went below the threshhold level of detection. In one study Jessen et al. JAMA 277:18 1437-8 1997, hydroxyurea and ddl were used in combination for up to 65 weeks, but the patients did not reach an undetectable level (defined as less than 500 copies per ml).
In a later study, a therapeutic effect was again found, but the amount of virus in the blood failed to reach 100 copies per milliliter, the threshhold level of detection for the test used in that study. See Lori et al Combination of a Drug Targeting the Cell with a Drug Targeting the Virus Controls Human Immunodeficiency Virus Type 1 Resistance, AIDS Research and Human Retroviruses vol. 13, Number 16, Mary Ann Liebert, Inc. (1997) Further, patients who were treated with both hydroxyurea and the reverse transcriptase inhibitor ddl had higher rates of formation of ddl resistant mutations than those treated with ddl alone, although the overall level of virus remained low and rather constant. An incident was reported where two individuals on hydroxyurea and ddl treatment for a year stopped treatment for a year, without rebound. However, the initial level of viral load for each of them was so low that others have doubted whether these individuals had an infection.
The inventors have now found that hydroxyurea in combination with a reverse transcriptase inhibitor alone can be used to reduce the level of viral load in the blood to undetectable levels (less than 500 copies per milliliter), and that such treatment can be sustained without rebound over long periods of time. This discovery yields an important tool, particularly for use in treating patients for whom treatment with protease inhibitors is impractical, but also in a generalized scheme of treatment for the disease.